Generation of Hydrofluoronickelacycles from Trifluoroethylene and Ni(0): Ligand Effects on Regio-/Stereoselectivity and Reactivity

Treatment of Ni(0) complexes <b>1a</b>–<b>e</b> with sub-atmospheric pressures of trifluoro­ethylene (TrFE) affords hydrofluoro­nickela­cyclopentanes L₂Ni­(C₄F₆H₂) <b>2a</b>–<b>e</b> (L = PPh₃, P­(O-<i>o</i>-tol)₃, PPh₂Me, PPhMe₂, PMe₃). Fluorine NMR analysis of <b>2a</b>–<b>e</b> demonstrates predominant formation of three (of the possible six) isomers upon oxidative cycloaddition of TrFE: the <i>cis</i> and <i>trans</i> head–tail isomers and the <i>trans</i> head–head isomer, where the CHF group is defined as the TrFE “head”. The respective ratios of L₂Ni­(C₄F₆H₂) isomers are influenced by the nature of L, with smaller phosphines favoring the thermodynamically preferred (from DFT calculations) <i>trans</i> head–head isomer (cf. 50% with PMe₃) and the largest affording small amounts of the tail–tail isomers. Lewis and Brønsted acids induce a surprising double C–F bond activation in <b>2c</b>–<b>d</b>, affording small functionalized hydrofluoro­alkenes. Interestingly, varying the acid employed dictates the organic product obtained from the head–tail isomers: BF₃·OEt₂ is selective for 1,1,2,3-tetrafluoro­cyclobutene, whereas Me₃SiOTf and <i>N,N</i>-dimethylanilinium bromide yield (<i>Z,E</i>)-1,1,3,4-tetrafluoro­butadiene as the major fluorinated product. Reaction intermediates were isolated, and possible pathways are discussed